An Enhanced Ground Proximity Warning System (“EGPWS”) monitors several flight instrumentation inputs and refers to a database testing for the presence of ground proximity threats in the path of the aircraft. It does this by obtaining a reliable positional fix by means of receipt of Global Positioning System (“GPS”) data augmented by other navigational data. Once the EGPWS obtains a reliable positional fix, the EGPWS refers to a terrain and obstacle database, augmented by an airport database (collectively “terrain”). Based upon the current position, the EGPWS identifies terrain hazards along the projected path of the aircraft.
The EGPWS identifies hazards along the projected path of the aircraft by defining a trio of parallel paths representing the projected flight path on the ground, known as the groundtrack, and a parallel path or offset track on either side of the groundtrack displaced by a distance known as a Look-Ahead Detection Offset. The EGPWS determines which data to recall from the database by determining which cells or sub-cells contain or touch points on any of the three lines. Additionally, the current practice is to extend the projected flight path outwardly by a small angle to detect and to warn of off-path terrain hazards that may be present. This splaying outward of the offset tracks enhances the pilot's recognition of hazards that, in the course of a turn, might present a terrain hazard. Side span is the term for this angle that splays the detection envelope. In current practice, neither the Look-Ahead Detection Offset nor the Side Span varies with positioning source accuracy.
The database in the EGPWS tiles the surface of the earth with database cells. These cells are defined in a manner to minimize variations in their size due to the curvature of the earth. Within each cell, the database subdivides the cells into sixteen sub-cells, each of which, in turn, the database divides into sixteen sub-sub-cells, depending upon the level of resolution of terrain information necessary to convey the hazards extant therein. The database stores the highest terrain altitude within a sub-sub-cell in association with that sub-sub-cell. The highest of the sixteen stored sub-sub-cell altitudes within a given sub-cell is stored in association with that sub-cell. Similarly, the highest of the sixteen contained sub-cell altitudes is stored in association with the cell. Depending upon the resolution setting of the EGPWS, each of the cells, sub-cells, or sub-sub-cells that the projected groundtrack and the two parallel tracks offset by the Look-Ahead Detection Offset, touches, the value stored in association with that cell is compared to the aircraft altitude. (Within this discussion, the terms cell and sub-cell refer to divisions within the database without regard to degrees of cell subordination divisions of cells, i.e., sub-cells may refer to any division of an existing division.) Where the value stored in association with an implicated sub-cell and the instantaneous altitude fall within preset limits in relation one to another, an alert will sound or flash on the display.
In current EGPWS, where the Look-Ahead Detection Offset is large compared to the database cell size, the three parallel paths might not implicate all of the sub-cells within the projected flight path. It is possible that where these three parallel paths are widely spaced and where the terrain differences are discontinuous and abrupt, a terrain obstacle will fall between the two of the three parallel paths, leaving that obstacle undetected.
With current EGPWS monitors, under low-level flight conditions, especially where mountainous terrain exists, off-path hazards might cause alarms that are not desirable. Additionally, where the positioning source accuracy is high, current EGPWS do not narrow the projected flight path by decreasing the Look-Ahead Detection Offset. Where the actual path is well-fixed, too great a Look-Ahead Detection Offset will result in off-path terrain causing alerts that will not be useful to the pilot. These are known as nuisance alerts.
If a nuisance alert is issued often enough, the operator may begin to ignore all EGPWS alerts, whether valid or not, thus compromising safety. The EGPWS would meet its objectives of safer flight if, rather than to ignore or to turn off the EGPWS, the EGPWS would use a narrower projection of the anticipated flight path. Terrain objects that are just outside of the narrower flight paths would not evoke alerts. With fewer alerts, the pilot would continue to attend to those remaining EGPWS alerts.
What is needed is a system that reduces nuisance alerts and that ensures that no terrain cells are missed within the Look-Ahead Detection Offset.